1. Field of the Invention
The present invention relates to a heating element at least including a heating portion in which a heater pattern is formed in a plate portion of a heat-resistant base member, and a power-supply-terminal portion in which a power-supply terminal is formed in the heat-resistant base member.
2. Description of the Related Art
A heater in which a line or foil of metal having a high melting point such as molybdenum or tungsten is wrapped around or bonded to a heat-resistant base member made of sintered ceramic such as alumina, aluminum nitride or zirconia has been used for heating semiconductor wafers in manufacturing steps of semiconductor devices.
However, such a heater has drawbacks of being prone to deform or vaporize because the heating element is made of metal, being short-life, and being complicated to assemble (see the pyrolytic graphite/pyrolytic boron nitride heater from Union Carbide Services provided in “Vacuum” No. 12, (33), p. 53). Furthermore, use of sintered ceramic for the heat-resistant base member causes a problem that the binder in the sintered ceramic becomes impurities.
Then, to prevent such deformation or scattering of impurities due to a heat cycle, a ceramic heater is developed. The ceramic heater has a heat-resistant base member of pyrolytic boron nitride (PBN) having high mechanical strength and enabling high-efficiency heating, and a conductive layer of pyrolytic graphite on the heat-resistant base member (for example, see the pyrolytic graphite/pyrolytic boron nitride heater from Union Carbide Services provided in “Vacuum” No. 12, (33), p. 53; published Japanese translations of PCT international publication for patent applications No. H08-500932; Japanese Patent Laid-open (Kokai) No. H05-129210; and Japanese Patent Laid-open (Kokai) No. H06-61335).
An example of a heating element of such a heater is shown in FIG. 4. A heating element 20 has at least a heating portion 20a in which a heater pattern 3a is formed on a plate-shaped heat-resistant base member 21, and a power-supply-terminal portion 20c in which power-supply terminals 3c are formed at the rim of the surface of the heat-resistant base member 21 on which the heater pattern is formed. A protection layer 4 covers the heater pattern 3a. To the power-supply terminal 3c a power-supply member or a power terminal 5 is connected.
However, pyrolytic graphite used for the heating body is prone to undergo corrosion due to oxidation. Pyrolytic graphite has also reactivity with high-temperature gases used in the heating process. For example, hydrogen gas changes pyrolytic graphite into methane gas. Therefore, there is a problem that remaining oxygen or high-temperature gases in the process environment corrodes pyrolytic graphite in the power-supply-terminal portion exposed for power supply, and the power-supply-terminal portion is short-life.
To solve the problem, an attempt to locate the power-supply-terminal portion away from the heating portion is made. For example, the following solution is suggested: a power-supply terminal is connected to a power-terminal member via a power-supply member having a heater pattern which produces heat by energization. Insulating ceramic such as PBN is used for a protection layer covering the heater pattern, thereby preventing overheating of the power-supply-terminal portion to increase longevity of the power-supply terminal (see Japanese Patent Laid-open (Kokai) No. H11-354260).
Furthermore, the following method is suggested: assembling the power-supply-terminal portion made of carbon with an assembly part and forming a protection layer (see published Japanese translations of PCT international publication for patent applications No. H08-500932; International Publication WO2004/068541).
However, such a heating element has protrusions on the heating surface. It is necessary to provide a space between the heating surface and an object to be heated, which hampers compact design of the heating element. In addition, a protection layer in the vicinity of a connected part assembled from plural components is apt to produce cracks through usage. A conductive layer begins to corrode from the cracks, which causes a problem to shorten the life of the heating element. Furthermore, when the heating element is used in an environment corroding boride such as using halide etching gas, there is a drawback that an outermost layer of boride lacks resistance to corrosion, and corrosion of the outermost layer shortens the life of the heating element.
Use of pyrolytic boron nitride for a material of a heat-resistant base member as mentioned above gives high mechanical strength and enables high-efficiency heating. However, pyrolytic boron nitride has high anisotropy and is apt to warp. Pyrolytic boron nitride is also expensive. Therefore, use of sintered boron nitride for the heat-resistant base member is also suggested (see Japanese Patent Laid-open (Kokai) No. H04-358074).
However, when sintered boron nitride is used for the heat-resistant base member, the base member lacks mechanical strength, and it is necessary to thicken the base member. In addition, an amount of heat escaping from the side of the base member is huge. Therefore, it is impossible to rise temperature of a test sample sufficiently, especially high temperature of more than 700° C.
In addition, an advanced ceramic heater with electrostatic chuck on a heater for holding a semiconductor wafer to be heated is suggested currently (see Japanese Patent Laid-open (Kokai) No. H05-129210; Japanese Patent Laid-open (Kokai) No. H06-61335; Japanese Patent Laid-open (Kokai) No. H04-358074; Japanese Patent Laid-open (Kokai) No. H05-109876). However, the heater does not have sufficient heat resistance.